1. Field of the Invention
The present invention relates to a height sensor and an air spring device and, more particularly, to a height sensor such as a vehicle height sensor, a level sensor, and a distance sensor for optically detecting the height of an automobile, a streetcar and other vehicle. The invention also relates to an air spring device incorporating this height sensor, such as an air spring for a vehicle and an air spring for vibration isolation and damping used in an industrial machine, a building or the like.
2. Description of the Related Art
Conventionally, a magnetic type sensor, an ultrasonic type sensor, an optical reflection type sensor, and the like have been proposed as a height sensor which can be incorporated in an air spring. However, the magnetic type sensor has a drawback in that the stroke of the air spring is limited depending on the structure of the air spring, while the ultrasonic type sensor has a drawback in that the cost is high. On the other hand, although most of the optical reflection type sensors are less affected by these drawbacks, they have a problem in that if a light reflection plate becomes contaminated with dirt or the like and its light reflectance varies greatly, the measured results also vary, thereby making it impossible to ensure precision.
For this reason, the present inventors have proposed a height sensor which overcomes the above-described drawbacks as disclosed in U.S. Pat. Ser. No. 07/924,467, now U.S. Pat. No. 5,229,829. As shown in FIG. 11, this height sensor comprises a pair of light-emitting elements constituted by a first light-emitting element 10 and a second light-emitting element 12. The first light-emitting element 10 and the second light-emitting element 12 are disposed at positions spaced at different distances from a surface to be measured 14 and are adapted to emit beams of light alternately. If it is assumed that the height of the first light-emitting element 10 from the surface to be measured 14 is x, the difference in height (an amount of offset) between the first light-emitting element 10 and the second light-emitting element 12 is X.sub.o, the luminous intensity of the first light-emitting element 10 is C.sub.1, the luminous intensity of the second light-emitting element 12 is C.sub.2, and the reflectance of the surface to be measured 14 is R, then the output E1 of a light-receiving element 16 at the time when the first light-emitting element 10 emits a beam is expressed by Formula (1) below, while the output E2 of the light-receiving element 16 at the time when the second light-emitting element 12 emits a beam is expressed by Formula (2) below. ##EQU1##
If the ratio of the output E1 to the output E2 is determined, it is expressed by Formula (3) below, and the output ratio El/E2 ceases to be affected by the reflectance R of the surface to be measured 14. ##EQU2##
Accordingly, even in a case where the reflectance R of the surface to be measured 14 varies due to the contamination and the outputs E1 and E2 decrease as shown in FIG. 12, it is possible to prevent the undesirable effects of the contamination of the surface to be measured 14 by using the output ratio El/E2. Accordingly, this height sensor is provided with a signal processor circuit 18 which comprises a logarithmic converter circuit, a filter, and a rectifier circuit. The first light-emitting element 10 and the second light-emitting element 12 are made to emit beams of light alternately, and the light-receiving element 16 receives the beams reflected from the surface to be measured 14 and outputs a signal corresponding to the quantity of light received. The signal outputted from the light-receiving element 16 is subjected to logarithmic conversion by the logarithmic converter circuit. The output of the logarithmic converter circuit has its direct current components removed by the filter, the direct current components being the signal components having a frequency lower than the light emission frequency signal of the light-emitting elements. The peak-to-peak value of the remaining alternating current components corresponds to a value obtained by subtracting the logarithmically converted value of the signal outputted from the light-receiving element 16 when the second light-emitting element 12 emits a beam, from the logarithmically converted value of the signal outputted from the light-receiving element 16 when the first light-emitting element 10 emits a beam, i.e., corresponds to the ratio between the signal outputted from the light-receiving element 16 when the first light-emitting element 10 emits a beam and the signal outputted from the light-receiving element 16 when the second light-emitting element 12 emits a beam. Accordingly, by rectifying this filter output, it is possible to obtain a signal corresponding to the height x of the first light-emitting element 10 from the surface to be measured 14.
However, with the above-described height sensor, since the pair of light-emitting elements are disposed with a horizontal distance L therebetween, that area of the reflection plate which the beams from both light-emitting elements can reach is relatively small. In particular, if the reflection plate, i.e., an area to be measured, approaches the light-emitting elements' side beyond the straight line A shown in FIG. 11, the area of light which can be received by the light-receiving element (the shadowed portion in the drawing) becomes very small, so that the amount of light received by the light-receiving element is reduced sharply. Accordingly, if the distance between the light-emitting elements and the reflection plate becomes smaller, it becomes difficult for the sensor to detect the distance.
In addition, since the pair of light-emitting elements need to be disposed by being spaced apart from each other in the heightwise direction by an amount of offset X.sub.o, there is a problem in that the heightwise dimension of the height sensor becomes long, with the result that the height sensor becomes relatively large in size. To overcome this problem, it suffices if the amount of offset X.sub.o is reduced, but since the effect of the reflectance of the light reflection plate is prevented by the amount of offset X.sub.o, the accuracy becomes deteriorated if the amount of offset X.sub.o is reduced.